Known methods for preparing dihydroxydiphenylmethane are as follows:
(1) a method comprising reacting phenol with dimethylol urea in the presence of an acidic catalyst and extracting the aimed dihydroxydiphenylmethane from the resulting product followed by recrystallization from water (cf. U.S. Pat. Ser. No. 2,617,832);
(2) a method comprising reacting phenol with formaldehyde and then silylating the resulting product followed by fractional distillation and desylilation (cf. U.S. Pat. Ser. No. 2,804,480);
(3) a method comprising reacting phenol with formaldehyde in dimethylformamide (DMF) [cf. JP-B-4838694 (The term "JP-B" as used herein means an "examined Japanese patent publication")]; and
(4) a method comprising reacting phenol with formaldehyde in the presence of urea, filtering the solid thus precipitated and recrystallizing the same from water (cf. JP-B-39-26844).
A reaction between phenol and formaldehyde in the presence of an acidic catalyst would generally give 10 to 25% by weight of 2,2'-dihydroxydiphenylmethane, which will be abbreviated as 2,2'-BPF hereinafter, 40 to 60% by weight of 2,4'-dihydroxydiphenylmethane, which will be abbreviated as 2,4'-BPF hereinafter, 25 to 40% by weight of 4,4'-dihydroxydiphenylmethane, which will be abbreviated as 4,4'-BPF hereinafter, and oligomers such as trimers, tetramers and higher ones, the total of 2,2'-BPF, 2,4'-BPF, 4,4'-BPF and oligomers being 100% by weight. No process for selectively synthesizing 4,4'BPF alone has been developed so far. Recently the demand for 4,4'-BPF has been more and more increasing. Therefore it is required to divide crude dihydroxydiphenylmethane, i.e., a mixture of 2,2'-BPF, 2,4'-BPF, 4,4'-BPF and oligomers, which will be abbreviated as BPF hereinafter, on an industrial scale.
However the above method (1) is disadvantageous from an industrial viewpoint, since the dimethylol urea employed as a starting material is unstable; the recrystallization from water gives only a limited yield; and the recovery of the phenol contained in the water, which is required during the course of the treatment of the waste, is not always easy. The method (2) is also disadvantageous from an industrial viewpoint, since it requires an expensive silylating agent. The selectivity of 4,4'-BPF established by the method (3) is somewhat improved but not yet sufficient. Further it is required in this method to recover the phenol as well as the expensive solvent. Furthermore, it is not shown whether highly pure 4,4'-BPF can be obtained or not, though a purification step, i.e., recrystallization is described. The method (4) is essentially similar to the method (1) and thus it is also disadvantageous from an industrial viewpoint. Thus none of these known methods enables the advantageous preparation of 4,4'-BPF on an industrial scale.